Last class

More mechanical testing

Fracture toughness - pre-cracked specimen

Creep - constant load, elevated temperature

Fatigue - constant load, S/N curves, endurance limit

Last class

More mechanical testing

Fracture toughness - pre-cracked specimen

Creep - constant load, elevated temperature

Fatigue - constant load, S/N curves, endurance limit

Fracture surfaces

Brittle vs. ductile

Crack origins, chevron markings

Beachmarks

Fracture surfaces

Brittle vs. ductile

Crack origins, chevron markings

Beachmarks

Today

Atom arrangements in metals

Dislocations

Today

Atom arrangements in metals

Dislocations

Crystal structures of materials - where are the atoms?

Polymers - mostly not crystallinestructure extremely complicated

Ceramic materials - complicatedmany different types of atom

arrangements

Crystal structures of materials - where are the atoms?

Polymers - mostly not crystallinestructure extremely complicated

Ceramic materials - complicatedmany different types of atom

arrangements

Metals - really simple - most have one of three typesMetals - really simple - most have one of three types

Crystal structures of pure metalsCrystal structures of pure metals

Most pure metals exhibit one of three types

1. cubic close packing (ccp or A1)2. hexagonal close packing (hcp or A3)3. A2 (almost universally referred to by

the confusing notation 'bcc')

Most pure metals exhibit one of three types

1. cubic close packing (ccp or A1)2. hexagonal close packing (hcp or A3)3. A2 (almost universally referred to by

the confusing notation 'bcc')

Crystal structures of pure metals

cubic close-packed (ccp)

close-packed plane of atoms

Crystal structures of pure metals

cubic close-packed (ccp)

close-packed plane of atoms

Crystal structures of pure metals

cubic close-packed (ccp)

ABCABC layer sequence close-packed plane of atoms

Crystal structures of pure metals

cubic close-packed (ccp)

ABCABC layer sequence close-packed plane of atoms

AB

C

A

AB

C

Crystal structures of pure metals

hexagonal close-packed (hcp)

Crystal structures of pure metals

hexagonal close-packed (hcp)

Zn, Cd, Co, Ti, Zr……Zn, Cd, Co, Ti, Zr……

close-packed plane of atoms

hexagonhexagon

Crystal structures of pure metals

hexagonal close-packed (hcp)

ABAB layer sequence close-packed plane of atoms

Crystal structures of pure metals

hexagonal close-packed (hcp)

ABAB layer sequence close-packed plane of atoms

AB

AB

AB

Fe, V, Cr, Mo, W, Ta……Fe, V, Cr, Mo, W, Ta……

Crystal structures of pure metals

A2 structure - so-called "bcc" metal structure

almost close-packed atom planes

Crystal structures of pure metals

A2 structure - so-called "bcc" metal structure

almost close-packed atom planes

some empty space some empty space

Dislocations

And now…the rest of the story (on plastic deformation)!

Dislocations

And now…the rest of the story (on plastic deformation)!

Mechanism of plastic deformation connected with existence of defects in atom arrangement known as dislocations

Mechanism of plastic deformation connected with existence of defects in atom arrangement known as dislocations

Ideally, atom arrangement within a crystal repeats perfectly Ideally, atom arrangement within a crystal repeats perfectly

Mistakes (defects) in repetition occur in reality Mistakes (defects) in repetition occur in reality

Dislocations

Situation is this:

strength of a material w/ no dislocations is

Dislocations

Situation is this:

strength of a material w/ no dislocations is

20-100 times greater than ordinary materials 20-100 times greater than ordinary materials

Think of edge dislocation as extra plane of atoms partially inserted into crystal

Think of edge dislocation as extra plane of atoms partially inserted into crystal

Dislocations

These things are real!

We can see them! (in an electron microscope)

Dislocations

These things are real!

We can see them! (in an electron microscope)

Dislocation line

0.00001 cm

Dislocations

Move under a shear stress

Dislocations

Move under a shear stress

Dislocations

Move under a shear stress

Dislocations

Move under a shear stress

Dislocations

Move under a shear stress

Dislocations

Move under a shear stress

Dislocations

Move under a shear stress

Dislocations

Move under a shear stress

Dislocations

Move under a shear stress

Dislocations

Move under a shear stress

Dislocations

Move under a shear stress

Dislocations

Move under a shear stress

Dislocations

Permanent change in shape results

Dislocations

Permanent change in shape results

Dislocation has disappearedDislocation has disappeared

Dislocations move along a slip planeDislocations move along a slip plane

Dislocations

Watch a real one move!

Dislocations

Watch a real one move!

QuickTime™ and aCinepak decompressor

are needed to see this picture.

Dislocations

If hundreds of thousands of dislocations move through material, microscopic steps produced in the surface as below

Dislocations

If hundreds of thousands of dislocations move through material, microscopic steps produced in the surface as below

Dislocations - Initial overviewDislocations - Initial overview

Material w/ NO dislocations is very strongMaterial w/ NO dislocations is very strong

Dislocations weaken a materialDislocations weaken a material

But it cannot be deformed plasticallyBut it cannot be deformed plastically

But dislocations make plastic deformation possibleBut dislocations make plastic deformation possible